Apparatus and method for synchronizing a cordless telephone base unit and multiple portable units on a common communication channel

ABSTRACT

A cordless telephone system achieves synchronism for each one of multiple portable units to the same communication channel for communicating with a common base unit. The cordless system uses a dual channel process to allow both the freedom to dynamically change channels as needed to avoid interference while simultaneously providing a capability to resynchronize any portable unit with the base unit on a selected active channel. This is achieved through the use of a fixed service channel and an active channel which changes as needed. If a channel change is made during a call, after the call is completed, the base unit returns to the previous active channel and broadcasts a message to the multiple portable units indicative of the new active channel. Also, any portable unit that attempts to communicate with the base unit on the wrong channel and fails to get a response will transmit a message on the fixed service channel requesting the current active channel from the base unit. Responsive to this request, the base unit provides information indicative of the active channel over the service channel. Once equipped with the current active channel information, the portable unit is able to reach and establish communications with the base unit.

CROSS-REFERENCE TO RELATED APPLICATION

This invention is related to U.S. Pat. No. 5,689,549, issuing to G.Bertocci et al. on Nov. 18, 1997, and for which the associated U.S.patent application Ser. No. 08/347,745, was filed concurrently with theU.S. patent application for the present invention, both applicationshaving been assigned to a common assignee.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to cordless telephones and, more particularly, toa cordless telephone having a plurality of portable units arranged forcommunicating with a base unit.

2. Description of the Prior Art

Cordless telephone systems presently available in the art includemultiple portable units that operate with a single base unit. An exampleof such a telephone system is described in U.S. Pat. No. 5,325,420.Although such cordless telephone systems exist in the art, the number ofportable units that operate easily with a single base is generallylimited, typically to two or three units. This is due primarily to awidely used architecture that requires the base unit to keep track ofthe number of portable units in the system.

The cordless telephone systems employing multiple portable units foroperation with a single base are required to somehow synchronize allportable units with the base unit on a common channel from a pluralityof channels for setting up calls, i.e., initiating communications to andreceiving communications from the base unit. Otherwise, by way ofexample, an incoming ringing signal received at the base unit andretransmitted to the portable units may not be received by all of theportable units. Rather, this ringing signal will be received only by theportable unit that is on the channel last used by the base unit, if forsome reason, the base unit and this portable unit moved away from thecommon channel. Other portable units in the system are thus not able toreach and communicate with the base unit.

One cordless telephone system available in the art employs a dedicatedcommon signaling channel to set up calls. Communication is initiated onthis dedicated channel and the base unit and the portable unit involvedin the communication both move to another suitable channel for voicecommunication. Since cordless telephones have been allocated a limitednumber of communication channels over which they may operate, however,this system has the disadvantage of further reducing the availablenumber of channels by minimally committing one of these as the commonsignaling channel.

Another cordless telephone system, in executing a channelsynchronization process, requires each portable unit to confirm that ithas received the latest or last current channel. This confirmationprocess limits the number of portable units that reasonable can be inthe system, however, since every time a channel change takes place eachportable unit in the system must respond. Thus the response time isappreciably degraded as additional portable units are added to thissystem.

Although the above-described cordless telephone systems wherein multipleportable units are able to communicate with a single base do providecordless telephone communications, they do so only with the describedlimitations. It is desirable therefore to have a cordless telephonewherein multiple portable units communicate with a single base withoutthese limitations.

SUMMARY OF THE INVENTION

In accordance with the invention, a cordless telephone system includesan arrangement which reliably achieves synchronism to the samecommunication channel for each one of multiple cordless telephoneportable units communicating with a common base unit.

In accordance with the disclosed embodiment of the invention, thearrangement includes a process which ensures channel synchronizationbetween all portable units and the base unit and provides a recoveryroutine should any portable unit lose such channel synchronization. Thisis achieved by having not only a current active channel which is usedfor establishing and maintaining ongoing communications between the baseunit and one of the multiple portable units, but also a service channelwhich the base unit monitors and the portable units go to when they areunsuccessful in communicating with the base unit on what was expected tobe the current active channel.

In the execution of the recovery routine, a portable unit contacts thebase unit on the service channel and the base unit responds on thisservice channel by providing data to the portable unit indicative of thecurrent active channel. Once equipped with the current active channelinformation, a portable unit is then able to reach and establishcommunications with the base unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its mode of operation will be more clearly understoodfrom the following detailed description when read with the appendeddrawing in which:

FIG. 1 shows a cordless telephone base unit and multiple portable unitsfor communicating with the base unit;

FIG. 2 is a functional block representation of a cordless telephone baseunit and portable unit both operative in accordance with the principlesof the present invention;

FIG. 3 shows the protocol of the cordless telephone depicting specificprocesses executed by a portable unit in establishing communicationswith the base unit, in accordance with the principles of the presentinvention;

FIG. 4 shows the protocol of the cordless telephone depicting thespecific processes executed by the base unit in establishingcommunications with one of the multiple portable units, in accordancewith the principles of the present invention;

FIG. 5 shows the protocol of the cordless telephone depicting additionalspecific processes executed by a portable unit in establishingcommunications with the base unit, in accordance with the principles ofthe present invention; and

FIG. 6 shows the protocol of the cordless telephone depicting additionalspecific processes executed by the base unit in establishingcommunications with one of the multiple portable units, in accordancewith the principles of the present invention.

Throughout the drawing, the same element when shown in more that onefigure is designated by the same reference numeral.

DETAILED DESCRIPTION

Referring now to FIG. 1 of the drawing, there is shown a cordlesstelephone base unit 10 which provides access to a telephone centraloffice (not shown) over tip-ring lines 101-102, and multiple portable orhandset units 20 through 40. Although only three handset units are shownand described herein, it is to be understood that a greater or fewernumber of handset units may be employed in practicing the principles ofthis invention.

In accordance with the disclosed embodiment, each of the handset units20 through 40 may securely access the base unit 10 and share a commontelephone line through this base unit. Although a radio frequency (RF)link is shown as existing only between base unit 10 and handset unit 20,such RF link may be established between the base unit and any of thehandset units 20, 30 or 40. Thus, incoming telephone calls received atthe base unit 10 from the central office may be answered by a person ateither of the handsets 20 through 40. Similarly, a user of either of thehandset units 20 through 40 may originate a call to go over the tip-ringlines 101-102 to the central office.

Referring next to FIG. 2, there is shown a block representation of themajor functional components of the base unit 10 and the handset unit 20,both depicted in FIG. 1 and operative over a plurality of communicationchannels. With regard to the functional components illustrativelydescribed in handset unit 20, handset units 30 and 40 incorporate thesesame components and are operationally identical.

Included in the base unit 10 is a control unit 120 which advantageouslyprovides a number of control functions. The control unit 120 generatessecurity code and frequency channel data, which includes data indicativeof an active channel and a service channel, during a registrationprocedure for subsequent use by the base unit 10 in communicating withthe handset unit 20. The active channel is the channel used by both thebase unit and the handset unit for normal voice communications and fortransmitting and receiving opcodes for executing identified functions.The service channel is the channel used by the base unit and the handsetunit for respectively transmitting and receiving information indicativeof the current active channel. Use of the dual channel process allowsboth the freedom to dynamially change channels as needed to avoidinterference while simultaneously providing a capability toresynchronize any handset unit with the base unit on the active channel.An arrangement suitable for use in selecting an active channel free ofinterference is described in U.S. Pat. No. 5,044,010 which issued to R.Frenkiel on Aug. 27, 1991, this patent being incorporated herein byreference.

In accordance with the inventive embodiment, the service channel and theactive channel may be one and the same. If, by way of example, inresponse to interference, the channel determined to be the clearestchannel is also the service channel, then this channel is thenautomatically selected by the base unit during a channel change routineas the active channel or, alternatively, inserted as the next channelfor selection as the active channel when a user manually performs thechannel change by pressing a suitably designated button. Thus, thenumber of communication channels over which the cordless telephone mayoperate is not limited in any way by employing both an active channeland a service channel. Rather, all communication channels remainavailable for voice communcations as long as they do not containinterference.

The security code data generated by control unit 120 must be communcatedto the control unit 220 in the handset unit 20 and to the control unitsin all the other handset units in order for the base unit to be able tocommunicate with these handset units and the handset units, in turn, tobe able to communicate with the base unit. During each registrationprocedure, the base unit chooses a random security code that remainsunchanged until a user performs a special key sequence or,alternatively, presses a dedicated button, to perform anotherregistration procedure, thereby changing the security code again. Theregistration data in both the base unit and handset units isadvantageously stored in nonvolatile memory to provide robustness incase of power loss or any other condition that might corrupt thisinformation.

In a first type of registration procedure, usable in accordance with thedisclosed embodiment, control unit 120 interfaces with a control unit220 in the handset unit 20 while the handset unit 20 is located in amating cradle in the base unit 10. In this registration procedure, boththe security code and frequency channel data is transferred to thecontrol unit 220 in accordance with the teachings of U.S. Pat. No.4,736,404 issued to R. E. Anglikowski, et al. on Apr. 5, 1988, thispatent being incorporated herein by reference. Once in the registrationmode, each handset unit is cradled to receive the same security code andfrequency channel information. After each handset unit is configuredwith this information, the base unit is returned to the normal mode ofoperation.

In a second type of registration procedure, usable in accordance withthe disclosed embodiment, the control unit 120 in the base unit 10communicates with the control unit 220 in the handset unit 20 after thehandset unit and base unit have been configured in a registration mode.While in this mode, the handset unit 20 is brought within closeproximity to the base unit 10 and the registration procedure executed.During this registration procedure, all handset units intended foroperation in the cordless telephone system are configured with the samesecurity code and frequency channel information. Advantageously, if itis desired to register an additional telephone handset unit with thebase unit at some subsequent time, this is also easily achievable withthis registration procedure. A registration procedure suitable for usein registering the base unit and handset unit while in close proximitywith each other is set forth in copending U.S. patent application Ser.No. 08/270,348 filed on Jul. 5, 1994, this application beingincorporated herein by reference.

This control unit 120 also compares the received security code data withits stored security code data during the establishing of a two-way RFcommunications link between the handset unit 20 and the base unit 10. Afavorable comparison of the data from the two security codes must beachieved in order for the base unit 10 to respond to arequest-for-service signal from a handset unit. This control unit 120also receives and processes opcode data provided by the handset unit 20for dialing and providing signaling information out to a central officeor other appropriate switch via a telephone circuit 121 and overtip-ring lines 101 and 102. Control unit 120 may be implemented throughthe use of a microcomputer containing ROM, RAM and through use of theproper coding. Such a microcomputer is known in the art and is readilyavailable from semiconductor manufacturers such as Signetics, Intel andAMD. The telephone circuit 121 serves as a "plain old telephone service"(POTS) interface for voice signals on the tip-ring lines 101 and 102which are sent to transmitter 122 and received from receiver 123. Aconventional power supply (not shown) provides operating power for allof the circuitry in the base unit 10.

In order for the base unit 10 to achieve effective coverage throughoutits operating range, a signal strength monitor circuit 126 continuallymonitors the strength of the received signal from the handset unit 20during ongoing communications with the handset unit 20. This signalstrength monitor circuit 126 may be, for example, a received signalstrength indicator (RSSI) circuit. This RSSI circuit produces an outputvoltage that is proportional to the strength of the received signal fromthe handset unit 20.

Responsive to the strength of the received signal from the handset unit20, as determined by the signal strength monitor circuit 126, thecontrol unit 120 regulates the amount of power transmitted by thetransmitter 122 to the handset unit 20. Thus, when the handset unit 20is in close proximity to the base unit 10, the level of power radiatedby the radio transmitter 122 is reduced to a minimum acceptable level.And when the handset unit 20 is determined to be located near the fringeof the telephone's operating range, the level of power radiated by radiotransmitter 122 is increased to its maximum permitted level.

Both the base unit 10 and the handset unit 20 are operable on aplurality of communication channels. The control unit 120 configures aradio transmitter 122 and a radio receiver 123 in the base unit 10 forproper operation on the active one of the plurality of channels whencommunicating with one of the plurality of handset units, for example,handset unit 20. The control unit 120 also provides the appropriatechannel control information to the handset unit 20 over the activechannel or, alternative, when necessary for the handset unit to locatethe active channel, over the service channel.

The transmit and receive signals of the base unit 10 are coupled to aduplexer 125 which permits the transmitter 122 and the receiver 123 toboth simultaneously operate over antenna 124 while preventing the outputof transmitter 122 from being coupled directly to the input of thereceiver 123. When the base unit is in an idle state awaiting anincoming telephone call or a request-for-service signal from a handsetunit, transmitter 122 is turned off while receiver 123 remains on todetect the request-for-service signal.

Referring next to the handset unit 20, there is shown the control unit220 which stores the security code data that is generated by the baseunit 10 and provided to the handset unit 20. This security code datastored in control unit 220 is transmitted from the handset unit 20 tothe base unit 10 while establishing initial communications through arequest-for-service signal as well as during the transfer of subsequentopcode data to the base unit. These signals are transmitted in afrequency shift keying (FSK) format and includes a synchronizing signalimmediately followed by a data field which includes the security codegenerated by the control unit 120. Like the control unit 120, thiscontrol unit 220 may be implemented through the use of a microcomputercontaining ROM, RAM and through use of the proper coding. Such amicrocomputer is known in the art and is readily available fromsemiconductor manufacturers such as Signetics, Intel and AMD.

Communications with the base unit 10 are provided via a radiotransmitter 222 and a radio receiver 223 in the handset unit 20. Theoutput of the transmitter 222 and input for the receiver 223 arecommonly coupled through a duplexer 225 to an antenna 224. The receiver223 demodulates voice signals transmitted by the base unit 10 andcouples these signals to an acoustical device such as, for example,loudspeaker 321. The transmitter 222 has as its input speech signalsfrom a microphone 230, security code data from control unit 220 andopcode data representative of entries on a keypad 228, all of which ittransmits to the base unit 10. The keypad 228 is used for entering dialdigits and control functions executable by the control unit 220 ortransmitted to the base unit 10. A signal strength monitor circuit 226is included in the handset unit 20 for continually monitoring thestrength of the received signal from the base unit 10 during ongoingcommunications with the handset unit 20. A battery 227 is also includedin the handset unit 20 for providing operating power for all circuitryin this unit.

A status indicator 229 is connected to the control unit 220 and providesan indication to the user of the handset unit when (1) a communicationslink has been established between the handset unit 20 and the base unit10 and (2) a communications link cannot be established due to thehandset unit being outside of the reception range of the base unit.

In order to conserve battery power, a controlled power up/power downmode of operation for the handset unit 20 is implemented in accordancewith the teachings of U.S. Pat. No. 4,731,814 issued to W. R. Becker etal. on Mar. 15, 1988. The battery 227 in the handset unit 20 is normallycharged while the handset unit is placed in a cradle located in, forexample, its dedicated base unit. Such a cradle may also be an auxiliarycharging cradle such as is described in United States Dec. 6, 1994, thispatent P. Yankura on Jul. 25, 1992, now abandoned this copendingapplication being incorporated herein by reference. When the handsetunit is removed from this base unit and is in an idle or standby stateawaiting a telephone call, power to the control unit 220, receiver 223and certain other selected circuitry in the handset unit 20 iscontrolled to minimize power consumption. Power to other non-essentialcircuitry in the handset unit 20 is turned completely off during thisstate. The handset unit automatically turns on to a full operating modefrom the controlled power up/power down mode in response to events suchas a user depressing a key on the keypad 228 or the receipt of a ringindication from a base unit, the ring indication being indicative of anincoming call directed to the handset unit.

Referring next to FIG. 3, there is shown a flow chart illustrating theoperation of the handset unit 20 with the desired functionality inproviding dial tone or some other service requested by a user at thetelephone handset unit. The functions in this flow chart areadvantageously provided by a process or program stored in ROM containedin control unit 220.

The process is entered in step 301 where the initializing parameters areset. These include generating a substitute security code which comprisesa randomly generated number having a sufficiently large number of digits(over 64 thousand different codes) so that other cordless telephonehandsets within the telephone system would be unlikely to have this samenumber generated for use as a security or identification code andthereby be able to interfere with an established communication linkbetween a handset unit and the base unit. As part of the initializingstep, the handset unit receiver is set on a first or activecommunication channel. It is this channel that the base unit 10 eitherlast used in communicating with this handset unit or otherwise informedthis handset unit that this channel was the active one selected fromamong a number of predetermined frequency channels for the handset unitto monitor. The handset unit 20 may be informed of the active channel bythe base unit via a second or service channel assigned by the base unitfor use by both the handset and base unit for synchronizing a handsetunit on the correct active channel, as described later herein withreference to FIG. 4.

The active channels number between one and "N" and are limited only bythe available number of allocated frequency channels allowed forcordless telephone use. In the United States, for example, there arepresently 10 duplex frequency channels available for use with cordlesstelephones that operate within the 46 through 50 MHz frequency band. Itis to be understood, however, that "N" could be any reasonable numberless than or considerably greater than 10 and the frequency band alsocould include the recently allocated 900 MHz band allocated for cordlesstelephones.

From step 301, the process advances to step 302 where a check is made todetermine the status of the system. When the base unit and a handsetunit establish a communication link, the base unit broadcasts a "systembusy" message to the other handset units. Upon receipt of the systembusy message, a flag is set inside each of these other handset units.From step 302, the process advances to decision 303 where if a systembusy flag has been set, the process advances to step 304. At this step304, the status indicator 229 provides a busy indication for viewing bya user of the handset unit. If a system busy flag has not been set inthe handset unit and detected in the decision 303, the process advancesto step 305 where a request-for-service signal is transmitted by thehandset unit on the active frequency channel. This signal includes asynchronizing signal and the security code provided by the base unit ina previous registration procedure performed between the base unit andthe handset unit.

The process next advances to decision 306 where it is determined if anacknowledge signal has been received by the handset unit receiver fromthe base unit. The acknowledge signal from the base unit must includethe same security code initially transmitted by the handset unit.Otherwise, the handset unit will not respond to the acknowledge signal.If the acknowledge signal is not received in a predetermined timeperiod, the process advances to step 307 where a request for the activechannel is transmitted by the handset unit on the service or secondcommunication channel. As earlier indicated herein, it is this channelthat is used by the base unit and handset unit for identifying thecurrent active channel. Any handset unit that attempts to communicatewith the base unit and fails to receive a response will transmit arequest on the fixed service channel requesting the current activechannel from the base unit. The active channel request is similarlyrequested in the process as it advances from step 304 to step 307. Sinceit is possible that a handset unit may have the wrong system busy statebecause it missed a "system-not-busy" message freeing the channel, thehandset unit also requests the identity of the active channel over theservice channel when the system busy flag is set.

From step 307, the process advances to decision 308 where adetermination is made as to whether the active channel information hasbeen received from the base unit. If this information has not beenreceived, the process is exited. If the active channel information hasbeen received, the process advances to step 309 where the handset unitis set to the active channel. From step 309, the process advances tostep 313 where a request-for-service signal is transmitted by thehandset unit on the active frequency channel. If an acknowledge signalis not received, the process is exited.

If the appropriate acknowledge signal is received at either decision 306or decision 314, a communications link between the handset unit and thebase unit is established with the base unit providing, for example, dialtone or intercom service to the handset unit as appropriate. From eitherdecision 306 or decision 314, the process advances to step 315 where thehandset unit then adopts for its further use and sends a new ortemporary substitute security code which comprises a randomly generatednumber selected by the handset unit. The base unit similarly adopts thissubstitute security code when received and, like the handset unit, usesit for all subsequent communications to the handset unit. Once the baseunit and handset unit have adopted this temporary substitute securitycode, neither the handset unit nor the base unit will respond to orexecute command functions from another otherwise similarly configuredcordless unit unaccompanied by this code. From step 315, the processadvances to step 316 where the handset unit provides an off-hookindication to the handset user so that the user knows thatcommunications has been established with the base unit.

Once the communications link is established between the handset unit anda base unit, the process advances to step 317, the first step in aseries of steps wherein user requests are processed. In step 317, thekeypad is scanned for the entry of a digit. In decision 318, any digitentered on the keypad is detected and this digit transmitted to the baseunit as reflected by step 319. The information transmitted to the baseunit in this step, as well as other steps in this part of the routine,is provided in the form of an opcode message and contains the temporarysubstitute security code from the handset unit and data representativeof the digit entered on the keypad

If the entry of a digit is not detected on the keypad in decision 318,the process advances to decision 320 which monitors the activation offunction keys. Function keys provide local enhancements and convenienceto the handset unit. Function keys on the handset unit include suchfunctions as switchhook flash, channel change, base page and redial, forexample. If the depression of a function key is detected by decision320, the process advances to step 321 where the function is executed.

If the depression of a function key is not detected by decision 320, theprocess advances to decision 323 which looks for a depression of theon-hook key reflecting that the user desires to terminate the existingcall. If this on-hook key has not been depressed, the process returns tostep 317 and repeats this routine. If this on-hook key has beendepressed, however, the process advances to step 324 where theappropriate opcode message is transmitted to the base unit, therebyreleasing it from this handset unit. Next the process advances to thestatus indicator step 325 where an on-hook indication is provided to theuser of the handset unit. This status indicator reflected in step 325 ismultifold in functionality and also provides an indication that acommunications link between the handset unit and a base unit cannot beestablished because the handset unit is outside of the reception rangeof a base unit. From the status indicator step 325, the process isexited.

Referring next to FIG. 4 there is shown a flow chart illustrating theoperation of the base unit 21 with the desired functionality inestablishing communications with a handset unit requesting some type oftelephone service. The functions in this flow chart are advantageouslyprovided by a process or program stored in ROM contained in control unit120.

The process is entered in step 401 where the active or selected firstcommunication channel is monitored by the base unit for receipt of arequest-for-service signal from one of the multiple handset units. Theprocess next advances to decision 402 where receipt of arequest-for-service signal is determined. If a request-for-servicesignal has not been received, the process advances to step 403 where theservice or selected second communication channel is monitored by thebase unit for receipt of a request-for-service signal from one of themultiple handset units. If a signal is received on this service channel,as determined by the decision 404, the process advances to step 405where the active or first communication channel information istransmitted to the requesting handset unit. From the step 405, theprocess returns to step 401.

If a request-for-service signal is received, as determined by thedecision 402, the process advances to decision 406 where the securitycode in the request-for-service signal is examined and determined to bevalid or invalid. If invalid, the process returns to step 401. If valid,the base unit then receives from the handset unit in step 407 the new ortemporary substitute security code which it stores in temporary memory,as provided in step 408, for use in subsequent communications with thehandset unit requesting service. This code is then transmitted on theactive channel in step 409 back to the handset unit requesting serviceso that this handset unit is informed that the base unit has adopted itssecurity code. With the successful completion of these steps, the baseunit is seized by the handset unit and operates as a dedicated base unitproviding dial tone or other requested service to the handset unit andestablishing a communications link over the active channel. From step409, the process advances to step 419 where a system-busy message isbroadcast to the other handset units using once again the commonsecurity code then shared by the base unit and the other handset unitsnot then involved in the communications link with the base unit.

Most communications over the established communications link between ahandset unit and the base unit is in the form of opcode messages andinclude the common temporary security code then shared by the base unitand the handset unit. As the base unit ignores any inadvertent messagesignals on its active channel that do not include this temporarysecurity code then associated with the handset unit, in the same manner,the handset unit ignores any inadvertent message signals on the activechannel that are not accompanied by its security code.

Once the communications link has been established, the process advancesfrom step 419 to step 410 where it monitors the active channel forreceipt of requested functions for execution. When a function request isreceived, the process advances to decision 412 where a determination ismade as to whether the function request is a special function request,which requires terminating the use of the new security code andreinserting the old security code, or a normal function request whichdoes not affect the security code setting in the base unit. Thus if thefunction request is a special one, the process advances to step 414where use of the new security code is terminated and the old securitycode reinstated. If the function request is a normal function request,however, the process advances to step 413 where the function request isexecuted.

By way of illustration of a special function request, one possibleopcode message from the handset unit to the base unit is a request toenter the standby mode which enables a telephone conversation to beswitched to a user at a second handset unit from a first handset unit.Thus upon receipt of this request, the base unit puts the telephone callthat it is then bridging between a first handset unit and the centraloffice on hold, drops the existing communications link and permits asecond handset unit to go-off hook and continue a conversation that wasstarted with this first handset unit. This is possible since in theexecution of this function request, the temporary security code isdropped and any handset unit, sharing the same security code receivedduring registration with the first handset unit, that goes off-hookwhile the base unit is held in its standby state, is able to obtainservice by executing this process with the base unit beginning with step401. For other normal function requests such as processing any digitreceived from the handset unit then in use with the base unit, thetemporary security code is not terminated, but rather remains in activememory for subsequent use by both the base unit and the handset unitwith which it has established communications. Such digit functionrequest is detected and translated to a dual-tone-multiple-frequencysignal in the base unit and then transmitted to the central office overthe tip-ring lines 101 and 102. After a normal function is executed instep 413, the process then returns to the routine at step 410.

If a function request is not received at step 411, the process advancesto decision 415 which looks for an on-hook opcode message reflectingthat the user desires to terminate the call. If this opcode message isnot received, the process returns to step 410 and repeats this routine.If this opcode message is received, however, the base unit is releasedand the process advances to step 416 where the new security code isreplaced in memory by the old security code. As part of this step, thebase unit also broadcasts the system-not-busy message to all of thehandset units. From step 416, the process advances to decision 417 wherea decision is made as to whether a new active channel has been selectedby a handset unit or the base unit because of, for example, disturbanceson a previously selected active channel. In order to inform all of thehandset units of the new active channel, a channel change command istransmitted in step 418 to all of the handset units informing theseunits of the new active channel. A channel change process suitable foruse herein is described in U.S. Pat. No. 5,044,010 which issued to R.Frenkiel et al., on Aug. 27, 1991, this patent being incorporated hereinby reference. From step 418, the process returns to step 401 where thebase unit once again begins to monitor the active channel and then theservice channel for a signal from any one of the multiple handset units.

Referring next to FIG. 5 there is shown a flow chart illustrating anadditional operation of the handset unit 20 with the desiredfunctionality. The functions in this flow chart are advantageouslyprovided by a process or program stored in ROM contained in control unit220.

The process is entered in step 501 where the active or selected firstcommunication channel is monitored by the handset unit for receipt of anincoming signal from the base unit. The process next advances todecision 502 where receipt of an incoming signal is determined. If anincoming signal has not been received, the process returns to step 501.If a signal is received, however, the process advances to step 503 wherea received security code is examined and determined to be valid orinvalid. If invalid, the process returns to step 501. If valid, however,the handset unit then adopts a new or temporary substitute security codein step 504 for its subsequent use in communicating with the base unitand also sends this code the base unit for its subsequent use incommunicating with this handset unit. From the step 504, the processadvances to step 505 where the call is processed in the manner describedin both the decisions and steps 316 through 321 as shown in FIG. 3, andearlier described herein. From step 505, the process advances todecision 506 which looks for a depression of the on-hook key reflectingthat the user desires to terminate the existing call. If this on-hookkey has not been depressed, the process returns to step 505 and the callcontinues to be processed in this step. If the on-hook key has beendepressed, however, the process advances to step 507 where use of thenew security code is terminated and in its place the old security codeis inserted for subsequent use in the handset unit. In this routine, theappropriate on-hook opcode message is also transmitted to the base unitthereby releasing it from this handset unit. From the step 507, theprocess returns to the step 501 where the active channel is monitoredfor an incoming signal from the base unit.

Referring next to FIG. 6 there is shown a flow chart illustrating anoperation of the base unit 21 with the desired functionality inresponding to an incoming telephone call. The functions in this flowchart are advantageously provided by a process or program stored in ROMcontained in control unit 120.

The process is entered in decision 601 where the base unit monitors thetip-ring lines 101 and 102 for an incoming telephone call. The processresides at decision 601 continually monitoring the tip-ring line for anincoming call. Once a call is detected at this decision, the processadvances to step 602 where a transmit ring code is generated over theactive channel by the base unit for receipt by the multiple handsetunits. This ring code comprises an opcode which also includes the commonsecurity code provided to the handset units by the base unit duringregistration of these units. From step 602, the process advances todecision 603 where the base unit examines the security code received inthe acknowledge signal from the handset unit and determines whether thissecurity code is valid or invalid. If invalid, the process returns tostep 601. If valid, the base unit receives from the handset unit in step604 the new or temporary substitute security code which it stores intemporary memory, as provided in step 605, for use in subsequentcommunications with the handset unit requesting service. This code isthen transmitted on the active channel in step 606 back to the handsetunit so that this handset unit is informed that the base unit hasadopted its security code. With the successful completion of thesesteps, the handset unit effectively answers the telephone call inresponse to the ring code generated and transmitted by the base unit.The base unit is thus seized by the responding handset unit and operatesas a dedicated base unit for this handset unit. From step 606, the baseunit broadcasts the system busy message to all other handset units usingthe original security code provided to all of the handset units duringthe registration process.

From step 612, the process advances to step 607 where the call isprocessed by the base unit in the manner as set forth in both steps anddecisions 410 through 418 described earlier herein with reference toFIG. 4. From step 607, the process advances to decision 608 where thecommunications link is monitored for an on-hook opcode signal from thehandset unit. If an on-hook opcode signal is not received, the processreturns to step 607. If an on-hook opcode signal is received, theprocess advances to step 609 where the new security code is terminatedin the base unit and the old security code reinserted as the active codein this unit. As a part of this step, the system-not-busy message isalso transmitted to all handset units. From step 609, the processadvances to decision 610 where it is determined if a new active channelhas been selected. If a new active channel has not been selected, theprocess returns to step 601. If, on the other hand, a new active channelhas been selected, the process advances to step 611. At this step 611, achannel change command is transmitted to all of the handset unitsinforming these units of the new active channel. From step 611, theprocess returns to step 601 where the base unit once again begins tomonitor the tip-ring lines for an incoming call from the central office.

Various other modifications of this invention are contemplated and mayobviously be resorted to by those skilled in the art without departingfrom the spirit and scope of the invention as hereinafter defined by theappended claims.

The invention claimed is:
 1. In a cordless telephone system employing aplurality of handset units and a base unit, an arrangement forestablishing communications between the base unit and each one of theplurality of handset units for communication with said base unit, thearrangement comprising:means for establishing voice communicationsbetween the base unit and one of the plurality of handset units over theinitial active communication channel; means in both the base unit andeach of the plurality of handset units for storing data indicative ofthe initial active communication channel and the service communicationchannel; means in the base unit, in response to conditions related tothe initial active communication channel, for selecting another one ofthe plurality of communications channels to be a substitute activecommunication channel, and storing data indicative of the selection ofsaid another one of the plurality of communications channels as thesubstitute active communication channel; means in each handset unit, inthe absence of establishing communications with the base unit over theactive communication channel, for contacting said base unit over theservice communication channel; and means in the base unit forcommunicating over the service communication channel data indicative ofthe substitute active communication channel, said handset using the datareceived from the base unit over the service communication channel forestablishing communications with the base unit over the substituteactive communication channel.
 2. The arrangement of claim 1 wherein theactive communication channels and the service communication channel arerandomly selected from the plurality of communications channels.
 3. Thearrangement of claim 1 wherein the active communication channels and theservice communication channel are selected from the plurality ofcommunication channels responsive to interference present on theplurality of communications channels.
 4. The arrangement of claim 3wherein the active communication channels and the service communicationchannel are selected from the plurality of communication channels havingthe longest period after which any interference last occurred thereon.5. The arrangement of claim 4 wherein the service communication channelselected from the plurality of communications channels is identical tothe substitute active communication channel selected from the pluralityof communication channels.
 6. In a cordless telephone system employing aplurality of handset units and a base unit, an arrangement forregistering a base unit and each one of the plurality of handset unitsfor communication with said base unit, the arrangement comprising:meansin said base unit for selecting an active communications channel and aservice communications channel from a plurality of communicationschannels, the active communication channel being a channel over whichsaid base unit establishes and maintains ongoing voice communicationswith one of the plurality of handset units, and the servicecommunication channel being a channel over which said base unitcommunicates the identity of the active communication channel to saidplurality of handset units: means for establishing communicationsbetween the base unit and each of the plurality of handset units forcoupling initialization data from said base unit to each of theplurality of handset units; and means in both the base unit and each ofthe plurality of handset units for storing said initialization data,said initialization data being indicative of the active communicationchannel and the service communication channel.
 7. The arrangement ofclaim 6 wherein said establishing means includes means for establishingan electrical path between said base unit and each of the plurality ofhandset units for coupling said initialization data.
 8. The arrangementof claim 6 wherein said establishing means includes means forestablishing a radio-frequency communication link between said base unitand each of the plurality of handset units for coupling saidinitialization data.
 9. The arrangement of claim 6 wherein the activecommunication channel and the service communication channel are randomlyselected from the plurality of communication channels.
 10. Thearrangement of claim 6 wherein the active communication channel and theservice communication channel are selected from the plurality ofcommunication channels responsive to interference present on theplurality of communication channels.
 11. The arrangement of claim 10wherein the active communication channel and the service communicationchannel are selected from the plurality of communication channels havingthe longest period after which any interference last occurred thereon.12. A method of establishing communications between a base unit and eachone of a plurality of handset units for communicating with said baseunit, the method comprising the steps of:selecting an initial activecommunication channel and a service communication channel from aplurality of communication channels; storing data indicative of both theinitial active communication channel and the service communicationchannel in both the base unit and each of the plurality of handsetunits; contacting the base unit by one of the plurality of handset unitsover the initial active communication channel for establishingcommunications between said base unit and said one of the plurality ofhandset units over the initial active communication channel; selectinganother one of the plurality of communications channels to be asubstitute active communication channel in response to conditionsrelated to the initial active communication channel, and storing dataindicative of the selection of said another one of the plurality ofcommunications channels as the substitute active communication channel;contacting said base unit over the service communication channel by saidone of the plurality of handset units, in the absence of establishingcommunications with the base unit over the initial active communicationchannel, for the purpose of receiving data indicating the selection forthe substitute active communication channel; and communicating to saidone of the plurality of handset units by said base unit data indicatingthe selection of the substitute active communication channel.
 13. Themethod pursuant to claim 12 wherein the active communication channelsand the service communication channel are randomly selected from theplurality of communication channels.
 14. The method pursuant to claim 12wherein the active communication channels and the service communicationchannel are selected from the plurality of communication channelsresponsive to interference present on the plurality of communicationchannels.
 15. The method pursuant to claim 14 wherein the activecommunication channels and the service communication channel areselected from the plurality of communication channels having the longestperiod after which any interference last occurred thereon.
 16. Themethod pursuant to claim 15 wherein the service communication channelselected from the plurality of communications channels is identical tothe substitute active communication channel selected from the pluralityof communication channels.
 17. A method of registering a base unit andeach one of a plurality of handset units in a cordless telephone forcommunicating with said base unit, the method comprising the stepsof:selecting an active communications channel and a servicecommunication channel from a plurality of communication channels;establishing communications between the base unit and each of theplurality of handset units for coupling initialization data from saidbase unit to each of the plurality of handset units; and; storing saidinitialization data in both the base unit and each of the plurality ofhandset units, said initialization data being indicative of both theactive communication channel and the service communication channel, theactive communication channel being a channel over which said base unitestablishes ongoing voice communications with one of the plurality ofhandset units, and the service communication channel being a channelover which said base unit communicates the identity of the activecommunication channel to said plurality of handset units.
 18. The methodpursuant to claim 17 wherein said establishing step includes the step ofestablishing an electrical path between said base unit and each of theplurality of handset units for coupling said initialization data. 19.The method pursuant to claim 17 wherein said establishing step includesthe step of establishing a radio-frequency communication link betweensaid base unit and each of the plurality of handset units for couplingsaid initialization data.
 20. The method pursuant to claim 17 whereinthe active communication channel and the service communication channelare randomly selected from the plurality of communication channels. 21.The method pursuant to claim 17 wherein the active communication channeland the service communication channel are selected from the plurality ofcommunication channels responsive to interference present on theplurality of communication channels.
 22. The method pursuant to claim 21wherein the active communication channel and the service communicationchannel are selected from the plurality of communication channels havingthe longest period after which any interference last occurred thereon.23. The method pursuant to claim 22 wherein the service communicationchannel selected from the plurality of communication channels isidentical to the active communication channel selected from theplurality of communication channels.
 24. In a cordless telephone systememploying a plurality of handset units and a base unit, an arrangementfor use by a handset unit in establishing communications with the baseunit, the arrangement comprising:means in the handset unit for storingdata indicative of both an initial active communication channel and aservice communication channel; means in the handset unit forestablishing communications with the base unit over the initial activecommunication channel; and means in the handset unit, in the absence ofestablishing communications with the base unit over the initial activecommunication channel, for contacting said base unit over the servicecommunication channel to receive data indicating the selection of asubstitute active communication channel, said base unit communicatingover the service communication channel to said handset unit dataindicative of the substitute active communication channel, said handsetusing the data received from the base unit over the servicecommunication channel to for establishing communications with the baseunit over the substitute active communication channel.
 25. Thearrangement of claim 24 wherein the active communication channels andthe service communication channel are randomly selected from theplurality of communication channels.
 26. The arrangement of claim 24wherein the active communication channels and the service communicationchannel are selected from the plurality of communication channelsresponsive to interference present on the plurality of communicationchannels.
 27. The arrangement of claim 26 wherein the activecommunication channels and the service communication channel areselected from the plurality of communication channels having the longestperiod after which any interference last occurred thereon.
 28. Thearrangement of claim 27 wherein the service communication channelselected from the plurality of communication channels is identical tothe substitute active communication channel selected from the pluralityof communication channels.
 29. The method pursuant to claim 12, furtherincluding the step of contacting said base unit over the substituteactive communication channel by said one of the plurality of handsetunits, wherein said one of the plurality of handset units uses the datareceived from the base unit over the service communication channel forestablishing communications with the base unit over the substituteactive communication channel.